CELL-PHONE TECHNOLOGY

Wireless phones which receive their signals from towers. A cell is typically the area (several
miles) around a tower in which a signal can be received.
Cell phones provide an incredible array of functions. Depending on the cell-phone model, you
can:
• Store contact information
• Make task or to-do lists
• Keep track of appointments and set reminders
• Use the built-in calculator for simple math
• Send or receive e-mail
• Get information (news, entertainment, stock quotes) from the internet
• Play games
• Watch TV
• Send text messages
• Integrate other devices such as PDAs, MP3 players and GPS receivers
A cell phone is a full-duplex device. That means that you use one frequency for talking and a
second, separate frequency for listening. Both people on the call can talk at once.
Division of a city into small cells allows extensive frequency reuse across a city, so that
millions of people can use cell phones simultaneously. Cell phones operate within cells, and they
can switch cells as they move around. Cells give cell phones incredible range. Someone using a
cell phone can drive hundreds of miles and maintain a conversation the entire time because of the
cellular approach. Each cell has a base station that consists of a tower and a small building
containing the radio equipment.

A single cell in an analog cell-phone system uses one-seventh of the available duplex voice
channels. That is, each cell is using one-seventh of the available channels so it has a unique set of
frequencies and there are no collisions:
• A cell-phone carrier typically gets 832 radio frequencies to use in a city.
• Each cell phone uses two frequencies per call -- a duplex channel -- so there are typically
395 voice channels per carrier. (The other 42 frequencies are used for control channels)
Therefore, each cell has about 56 voice channels available. In other words, in any cell, 56 people
can be talking on their cell phone at one time. Analog cellular systems are considered
first-generation mobile technology, or 1G. With digital transmission methods (2G), the number
of available channels increases. For example, a TDMA-based digital system (more on TDMA
later) can carry three times as many calls as an analog system, so each cell has about 168
channels available.
Cell phones have low-power transmitters in them. Many cell phones have two signal strengths:
0.6 watts and 3 watts. The base station is also transmitting at low power. Low-power transmitters
have two advantages:
• The transmissions of a base station and the phones within its cell do not make it very
far outside that cell. Therefore, 2 different cells can reuse the same 56 frequencies.
Hence, the same frequencies can be reused extensively across the city.
• The power consumption of the cell phone, which is normally battery-operated, is
relatively low. Low power means small batteries, and this is what has made handheld
cellular phones possible.
The cellular approach requires a large number of base stations in a city of any size. A typical
large city can have hundreds of towers. But because so many people are using cell phones, costs
remain low per user. Each carrier in each city also runs one central office called the Mobile
Telephone Switching Office (MTSO). This office handles all of the phone connections to the
normal land-based phone system, and controls all of the base stations in the region.

All cell phones have special codes associated with them. These codes are used to identify the
phone, the phone's owner and the service provider. The various Cell Phone Codes used are as
follows:
1. Electronic Serial Number (ESN) : It is a unique 32-digit number programmed into the
phone when it is manufactured.
2. Mobile Identification Number (MIN) : A 10-digit number derived from the phones
number
3. System Identification Code (SID) : A unique 5-digit number that is assigned to each
carrier by the Federal Communications Commission (FCC).
ESN is a permanent part of the phone while both MIN and SID codes are programmed into
the phone when a service plan is purchased and the phone is activated.
Some of the necessary terminologies for cell-phone connection are described:
1. Mobile Telephone Switching Office (MTSO) : The switching office that all base
station cell sites connect to. It is a sophisticated computer that monitors all cellular calls,
keeps track of the location of all cellular-equipped vehicles traveling in the system,
arranges hand-offs, keeps track of billing information, etc. The MTSO in turn interfaces
to the PSTN by connection to a Control Office.
2. Public Switched Telephone Network (PSTN) : It is the network of the world's public
circuit-switched telephone networks, in much the same way that the Internet is the
network of the world's public IP-based packet-switched networks. Originally a network
of fixed-line analogue telephone systems, the PSTN is now almost entirely digital, and
now includes mobile as well as fixed telephones.
3.
If you have a cell phone, you turn it on and someone tries to call you. Here is what happens to
the call:
• When you first power up the phone, it listens for an SID on the control channel. The
control channel is a special frequency that the phone and base station use to talk to one
another about things like call set-up and channel changing. If the phone cannot find any
control channels to listen to, it knows it is out of range and displays a "no service"
message.
• When it receives the SID, the phone compares it to the SID programmed into the phone.
If the SIDs match, the phone knows that the cell it is communicating with is part of its
home system.
• Along with the SID, the phone also transmits a registration request, and the MTSO
keeps track of your phone's location in a database -- this way, the MTSO knows which
cell you are in when it wants to ring your phone.
• The MTSO gets the call, and it tries to find you. It looks in its database to see which
cell you are in.
• The MTSO picks a frequency pair that your phone will use in that cell to take the call.
• The MTSO communicates with your phone over the control channel to tell it which
frequencies to use, and once your phone and the tower switch on those frequencies, the
call is connected. Now, you are talking by two-way radio to a friend.
• As you move toward the edge of your cell, your cell's base station notes that your signal
strength is diminishing. Meanwhile, the base station in the cell you are moving toward
(which is listening and measuring signal strength on all frequencies, not just its own
one-seventh) sees your phone's signal strength increasing. The two base stations
coordinate with each other through the MTSO, and at some point, your phone gets a
signal on a control channel telling it to change frequencies. This hand off switches your
phone to the new cell.

If you're on the phone and you move from one cell to another -- but the cell you move into is
covered by another service provider, not yours. Instead of dropping the call, it'll actually be
handed off to the other service provider.
If the SID on the control channel does not match the SID programmed into your phone, then the
phone knows it is roaming. The MTSO of the cell that you are roaming in contacts the MTSO of
your home system, which then checks its database to confirm that the SID of the phone you are
using is valid. Your home system verifies your phone to the local MTSO, which then tracks your
phone as you move through its cells. All of this happens within seconds.
On most phones, the word "roam" will come up on your phone's screen when you leave your
provider's coverage area and enter another's. If you want to roam internationally, you'll need a
phone that will work both at home and abroad. Different countries use different cellular access
technologies.

ELECTROMAGNETIC INTERFERENCE

Most of us experience electromagnetic interference on a fairly regular basis. For example:
• If you put your cell phone down on your desk near the computer, you can hear loud
static in your computer's speakers every time the phone and the tower handshake. In the
same way, your car's stereo produces loud static whenever you make a call on your cell
phone.
• When you dial a number on your home's wireless phone, you can hear the number being
dialed through the baby monitor.
• It is not uncommon for a truck to go by and have its CB radio overwhelm the FM station
you am listening to.
• Most of us have come across motors that cause radio or TV static.



None of these things, technically, should be happening. For example, a truck's CB radio is not
transmitting on the FM radio bands, so your radio should never hear CB signals. However, all
transmitters have some tendency to transmit at lower power on harmonic side bands, and this is
how the FM radio picks up the CB. The same thing holds true for the wireless phone crossing
over to the baby monitor. In the case of the cell phone affecting the computer's speakers, the wire
to each speaker is acting like an antenna, and it picks up side bands in the audible range.
These are not dire problems -- they are just a nuisance. But notice how common they are. In an
airplane, the same phenomena can cause big trouble.
An airplane contains a number of radios for a variety of tasks. There is a radio that the pilots use
to talk to ground control and air traffic control (ATC). There is another radio that the plane uses
to disclose its position to ATC computers. There are radar units used for guidance and weather
detection, and so on. All of these radios are transmitting and receiving information at specific
frequencies. If someone were to turn on a cell phone, the cell phone would transmit with a great
deal of power (up to 3 watts). If it happens to create interference that overlaps with radio
frequencies the plane is using, then messages between people or computers may be garbled. If
one of the wires in the plane has damaged shielding, there is some possibility of the wire picking
up the phone's signals just like my computer's speakers do. That could create faulty messages
between pieces of equipment within the plane.

Many hospitals have installed wireless networks for equipment networking. For example, in
case of a heart monitor, the black antenna sticking out of the top of the monitor connects it back
to the nursing station via a wireless network. If you use your cell phone and it creates
interference, it can disrupt the transmissions between different pieces of equipment. That is true
even if you simply have the cell phone turned on -- the cell phone and tower handshake with
each other every couple of minutes, and your phone sends a burst of data during each handshake.
The prohibition on laptops and CD players during takeoff and landing is addressing the same
issue, but the concerns here might fall into the category of "better safe than sorry." A poorly
shielded laptop could transmit a fair amount of radio energy at its operating frequency, and this
could, theoretically, create a problem.

MULTI-BAND VS. MULTI-MODE CELL-PHONES

1. Multiple band - A phone that has multiple-band capability can switch frequencies. For
example, a dual-band TDMA phone could use TDMA services in either an 800-MHz or
a 1900-MHz system. A quad-band GSM phone could use GSM service in the 850-MHz,
900-MHz, 1800-MHz or 1900-MHz band.



2. Multiple mode - In cell phones, "mode" refers to the type of transmission technology
used. So, a phone that supported AMPS and TDMA could switch back and forth as
needed. It's important that one of the modes is AMPS -- this gives you analog service if
you are in an area that doesn't have digital support.

3. Multiple band/Multiple mode - It allows you to switch between frequency bands and
transmission modes as needed.

Changing bands or modes is done automatically by phones that support these options. Usually
the phone will have a default option set, such as 1900-MHz TDMA, and will try to connect at
that frequency with that technology first. If it supports dual bands, it will switch to 800 MHz if it
cannot connect at 1900 MHz. And if the phone supports more than one mode, it will try the
digital mode(s) first, then switch to analog.
You can find both dual-mode and tri-mode phones. The term "tri-mode" can be deceptive. It
may mean that the phone supports two digital technologies, such as CDMA and TDMA, as well
as analog. In that case, it is a true tri-mode phone. But it can also mean that it supports one
digital technology in two bands and also offers analog support.

ANALOG CELL-PHONES (FIRST GENERATION)

In 1983, the analog cell-phone standard called AMPS (Advanced Mobile Phone System) was
approved by the FCC and first used in Chicago. AMPS uses a range of frequencies between 824
megahertz (MHz) and 894 MHz for analog cell phones. In order to encourage competition and
keep prices low, the U. S. government required the presence of two carriers in every market,
known as A and B carriers. One of the carriers was normally the local-exchange carrier (LEC),
a fancy way of saying the local phone company.
Carriers A and B are each assigned 832 frequencies: 790 for voice and 42 for data. A pair of
frequencies (one for transmit and one for receive) is used to create one channel. The frequencies
used in analog voice channels are typically 30 kHz wide -- 30 kHz was chosen as the standard
size because it gives you voice quality comparable to a wired telephone.


The transmit and receive frequencies of each voice channel are separated by 45 MHz to keep
them from interfering with each other. Each carrier has 395 voice channels, as well as 21 data
channels to use for housekeeping activities like registration and paging.
A version of AMPS known as Narrowband Advanced Mobile Phone Service (NAMPS)
incorporates some digital technology to allow the system to carry about three times as many
calls as the original version. Even though it uses digital technology, it is still considered analog.
AMPS and NAMPS only operate in the 800-MHz band and do not offer many of the features
common in digital cellular service, such as e-mail and Web browsing.

DIGITAL CELL-PHONES (SECOND GENERATION)

They use the same radio technology as analog phones, but they use it in a different way. Analog
systems do not fully utilize the signal between the phone and the cellular network -- analog
signals cannot be compressed and manipulated as easily as a true digital signal. Digital phones
convert your voice into binary information (1s and 0s) and then compress it. This compression
allows between three and 10 digital cell-phone calls to occupy the space of a single analog call.
Many digital cellular systems rely on frequency-shift keying (FSK) to send data back and forth
over AMPS. FSK uses two frequencies, one for 1s and the other for 0s, alternating rapidly
between the two to send digital information between the cell tower and the phone. Clever
modulation and encoding schemes are required to convert the analog information to digital,
compress it and convert it back again while maintaining an acceptable level of voice quality.
Hence, digital cell phones have to contain a lot of processing power.

INSIDE A CELL-PHONE

A basic digital cell phone contains just a few individual parts:

• A circuit board containing the brains of the phone
• An antenna
• An Liquid Crystal Display (LCD) screen
• A keyboard
• A microphone
• A speaker
• A battery
The circuit board is the heart of the system and contains several chips. The analog-to-digital and
digital-to-analog conversion chips translate the outgoing audio signal from analog to digital and


the incoming signal from digital back to analog. The digital signal processor (DSP) is a highly
customized processor designed to perform signal-manipulation calculations at high speed. The
microprocessor handles all the functions for the keyboard and display, deals with command and
control signaling with the base station and also coordinates the rest of the functions on the
board.The Read Only Memory (ROM) and Flash Memory chips provide storage for the phone's
operating system and customizable features, such as the phone directory. The Radio Frequency
(RF) and power section handles power management and recharging, and also deals with the
hundreds of FM channels. Finally, the RF amplifiers handle signals traveling to and from the
antenna.
The display has grown considerably in size as the number of features in cell phones have
increased. Most current phones offer built-in phone directories, calculators and games. And
many of the phones incorporate some type of PDA or Web browser. Some phones store certain
information, such as the SID and MIN codes, in internal Flash memory, while others use external
cards. Cell phones have tiny speakers and microphones.

CELL-PHONE TOWER

A cell-phone tower is typically a steel pole or lattice structure that rises hundreds of feet into the
air. The box houses the radio transmitters and receivers that let the tower communicate with
the phones. The radios transmitters and receivers connect with the antennae on the tower through
a set of thick cables. The tower and all of the cables and equipment at the base of the tower are
heavily grounded.



HOW VIBRATOR WORKS IN CELLPHONE

If you have a cell phone or a pager, then you know that having it ring in the middle of a movie or
performance is enough to get you killed in some cities. Vibrating devices that quietly replace the
ringer are therefore life-saving devices that are an important part of urban survival!
Figure below shows the inside of a small toy which vibrates heavily similar to a cellphone
device.

Inside the control unit is a small DC motor which drives the gear. Attached to the gear, there is a
small weight. This weight is about the size of a stack of 5 U.S. nickels, and it is mounted
off-center on the gear. When the motor spins the gear/weight combination (at 100 to 150 RPM),
the off-center mounting causes a strong vibration. Inside a cell phone or pager there is the same
sort of mechanism in a much smaller version.